I wouldnt suggest a XION PSU, they arent good quality units, and a 1kW unit for $100 should tell you something isnt right with it. A good 500-550W unit like the Seasonic S12II 520W PSU is more than capable of running a single 5870. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
You dont need a 1kW PSU, its silly to waste money on a PSU that is well above what you have any use for, what you are going to end up doing is getting a crappy one because you cant spring for a high end one like the Antec true power quatro or the corsair HX1000 which cost close to $200.
You have no need in the forseeable future for a 1kW PSU, a good 750W unit can support 2 5870s, unless you plan on running 3 GTX 480s soon to spend money on a 1kW unit is an absolute waste.
The 5870 is on par with the 4870x2 and the GTX 295, its slightly below the GTX 480. It wont be able to max crysis out at 1080P but nothing will, it will be able to max out most other games at that resolution and will let you play crysis on high settings.
Maxing out crysis requires twice the graphics hardware to max out anything else, you will need a dual card setup which your board likely wont support. I dont know why people always use crysis as their baseline, its not a good thing to strive to max out, if you can play crysis on high you can max most other things out.
2 5870s would be able to play crysis on very high but I doubt your dell could fit it in the case, the motherboard likely doesnt support crossfire, and thats a lot of heat for a small OEM case to handle. Its also a lot more than you need for any other game so unless you plan to mainly spend your time playing crysis its not a very good plan to spend extra money just to be able to max that single game out.
Starting with a crossfire configuration isnt the best idea, a single 5870 will handle most games on high or highest just fine and only starting with a single one will let you add a second one down the line if you decide you want more performance or you can swap it out for a single more powerful card, but if you start with two your only option in the future if you need more performance is to get a more powerful and more expensive card.
Start with a single 5870 but allow yourself crossfire capabilities so you can do it in the future if you want more power, but i think you will be perfectly happy with a single 5870 for a while.